1. Field of the Invention
The present invention relates to a novel bifunctional polymerizable compound, a liquid crystal composition containing it, an optical anisotropic material obtained by polymerizing the liquid crystal composition and an optical element.
2. Discussion of Background
Liquid crystal is roughly classified into thermotropic liquid crystal and lyotropic liquid crystal. The former itself forms a liquid crystal phase in a specific temperature range, and at a temperature higher than the temperature range, the liquid crystal is an isotropic liquid in a state where the respective molecules are randomly aligned, and it is a solid crystal at a temperature lower than the above range.
The thermotropic liquid crystal is classified into nematic liquid crystal, cholesteric liquid crystal, smectic liquid crystal, etc. The cholesteric liquid crystal comprises a pile of many layers, and in each thin layer, liquid crystal molecules are aligned so that their major axes are in parallel with the layer and their directions are in order. The directions of the molecules vary little by little by adjacent layers and as a whole, a helical structure is formed. Therefore, the liquid crystal molecules show specific optical characteristics. Specifically, the liquid crystal molecules are aligned such that they are twisted helically and as a result, either one of left and right circularly polarized light components is selectively reflected depending upon the helical pitch. For example, the transmittance of the cholesteric liquid crystal is measured using a circularly polarized light which is selectively reflected, a transmission spectrum having a steep wavelength dependence, i.e. a spectrum having a rectangle in the wavelength region with selective reflection, is obtained. This property can be applied to a mirror which reflects light at a specific wavelength, refractive index anomalous dispersion utilizing reflection, etc. Accordingly, the cholesteric liquid crystal is utilized for various optical elements. For example, JP-A-2005-209327 discloses a polarizing diffraction element constituted by using a cholesteric liquid crystal.
The origin of the word of the cholesteric liquid crystal is that many cholesterol esters belong thereto. The reason of the twisted structure is considered to be because molecules have structural asymmetry called optical activity (chirality) and accordingly when the molecules are overlaid, they form gentle distortion in the spatial arrangement. A commonly used cholesteric liquid crystal is obtained by adding a chiral dopant having e.g. a center of asymmetry to a common relatively low molecular weight nematic liquid crystal or smectic liquid crystal. However, with respect to such a common relatively low molecular weight cholesteric liquid crystal, when influenced by the surrounding temperature change, the helical pitch varies and as a result, the above reflection properties vary and thus the selective reflection wavelength varies. Further, the twisted alignment structure of the liquid crystal molecules are less stable, and the alignment state may be disturbed when subjected to an impact in some cases, such being problematic in formation of an optical element.
Considering favorable temperature characteristics and high reliability in formation of an optical element, suitable is a polymer type cholesteric polymer liquid crystal with excellent reliability. A cholesteric polymer liquid crystal is obtained by polymerizing a cholesteric liquid crystal composition comprising a nematic or smectic liquid crystalline compound having polymerizable functional groups and a chiral dopant having a polymerizable functional group.
However, of the polymerizable cholesteric liquid crystal composition, subtle disturbance and crystals in alignment of the liquid crystal occur by polymerization, and fluctuations in characteristics and light scattering derived therefrom occurs. As a result, in many cases, the rectangle of the selective reflection varies. Specifically, the steep wavelength dependence is impaired, and the rectangle spectrum is broadened. That means, from the viewpoint of the transmittance in the vicinity of the selective reflection region, the transmittance is decreased. That is, the light utilization efficiency can hardly be secured in an optical element utilizing the transmitted light at a wavelength in the vicinity of the selective reflection region.
Further, with respect to an optical element in recent years, the increase in intensity of light to be used and use of short wavelength light are in progress. Accordingly, a polymer type cholesteric polymer liquid crystal is required to have favorable light resistance, particularly not to impair light resistance of an optical element against blue light (wavelength: 300 nm to 450 nm). When a material inferior in light resistance is used for an optical element, a decrease in the transmittance and an increase in the optical distortion occur with time, and stable use over a long period of time will be difficult.
Accordingly, it is preferred that the polymer type cholesteric polymer liquid crystal does not contain a structure and a moiety which may decrease light resistance in its molecular structure as far as possible. Specifically, it is required to design the liquid crystal so as not to contain a structure which absorbs light at a relatively short wavelength, such as blue light, among lights to be used for an optical element, and is deteriorated. For example, JP-A-9-211409 discloses a monomer containing a terphenyl structure having three 1,4-phenylene groups connected in its molecule. However, in a case where a polymer type cholesteric polymer liquid crystal is formed by using such a monomer, a polymer liquid crystal to be obtained contains a terphenyl structure which is likely to absorb a short wavelength light and to be deteriorated. Accordingly, it is easily estimated that realization of favorable light resistance of an optical element against a blue light is difficult.